But some people develop such an intense obsession that they are left housebound, and may refuse to undergo potentially life-saving medical examinations.

They may deny themselves fluids, which can harm the kidneys, or take drugs to avoid "accidents".

Many sufferers will not even take a job if a toilet is located off a communal area and they can be observed going in or out.

And routine situations requiring the provision of a urine sample fill some patients with terror.

Experts believe that the stigma surrounding the phobia means that many people refuse to admit they have a problem.

The National Phobics Society (NPS) hopes its new campaign will go some way to tackling this issue.

It also argues the medical profession needs educating about toilet phobia in order to encourage sufferers to come forward.

Taboo

Nicky Lidbetter, NPS manager, said: "Few people will talk about having an anxiety disorder in the first place, but for them to admit that they have a toilet-related phobia is rare because of the obvious embarrassment and humiliation of being laughed at or not being taken seriously.

"But, no matter how funny we might find it, it's certainly no laughing matter. We have to tackle this condition head on."

Professor Paul Salkovskis, a leading clinical psychologist, said part of the problem was society's squeamishness about going to the toilet.

"Around the world we use a lot of humour and euphemism to describe what is a basic human function.

"We say 'I'm going to the bathroom' or 'I'm going to powder my nose' because there is a taboo surrounding using the toilet."

Several conditions are thought to be behind toilet phobia. These include:

Obsessive Compulsive Disorder (OCD) linked to a fear of contamination

Agoraphobia - an anxiety disorder commonly - and wrongly - linked to fear of open spaces, but which the National Phobics Society says is often manifest as a fear of feeling trapped, and a need to escape

Paruresis ('shy bladder' syndrome) - the fear of urinating in the company of others

Parcopresis ('bashful bowel' syndrome) - the inability to defecate in public toilets

Treatments include cognitive behaviour therapy, which helps people to break the cycle of faulty thinking, and hypnosis.